Would this be harmful to fish/snails/shrimp? I don't have fish at the moment but I'm doing my fishless cycle. There's nothing for them to feed on unless they eat dead plants.Remove that one manually and see if more show up. To kill of planaria you're going to have to use some kind of dewormer. A lot of people use dog dewormers. I have never personally had to get rid of planaria, but there are tutorials on using dog dewormer to get rid of them. Fenbendazole is the ingredient you should be looking for.